Carbon--carbon composites containing poorly graphitizing pitch as a binder and/or impregnant

ABSTRACT

A unitary composite structure having improved flexural strength and a reduced coefficient of thermal expansion comprising a heterogeneous combination of a carbonaceous reinforcing material interbonded with a matrix material, wherein the said matrix material is a poorly graphitizing carbonaceous pitch containing polymerized and cross-linked aromatic components is disclosed. Graphite electrodes comprised of the poorly graphitized pitch matrix material acting as a binder and/or an impregnant are also disclosed. Processes for the preparation of the poorly graphitizing pitch, the composite structure, and particularly the graphite electrodes are disclosed as well.

This application is a Division of prior U.S. application: Ser. No06/790,236 Filing Date Oct. 22, 1985, now U.S. Pat. No. 5,413,738.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of carbon-carbon composites and ismost applicable to graphite electrodes. More specifically, the presentinvention relates to a poorly graphitizing pitch and carbon-carboncomposites containing such pitch in the form of a binder, an impregnant,or both, which composites have improved flexural strength and reducedcoefficients of thermal expansion.

2. Discussion of Related Art

Carbon-carbon composites are well known and have found commercial use inmany different applications in the aerospace, chemical, electrical,metallurgical, nuclear, and other industries.

The popularity of these composites can be attributed to their goodmechanical properties as well as their ability to withstand extremelyhigh temperatures and pressures.

A short description of various patents relating to carbon-carboncomposites and their applications is set forth in a book entitled"Carbon and Graphite Fibers" edited by Marshall Sitting and published byNoyes Data Corp., New Jersey (1980). In a book entitled "Technology ofCarbon and Graphite Fiber Composites" by John Delmonte, published by VanNostrand & Reinhold Company, New York (1982), a review of the technologyof carbon-carbon composites is set forth. Processes for producing suchcarbon-carbon composites are discussed in the book entitled "Handbook ofComposites", Vol. 4, edited by A. Kelly and S. T. Mileiko, published byElsevier Science Publishers B. V., Holland (1983).

Generally, a carbon-carbon composite comprises a heterogeneouscombination of carbon reinforcing material interbonded with a carbonmatrix material. The carbonaceous reinforcing material can comprisecarbon or graphite fibers, carbon or graphite particles, andcombinations thereof. The carbon matrix material can be derived frompitch, organic resin, the thermal pyrolysis of a carbon-bearing vapor,and combinations thereof. As used herein, the term "carbon matrixmaterial" or more simply "matrix material" or "matrix" is understood tomean a material which acts as a binder, an impregnant, or both in thecontext of a carbon-carbon composite where such matrix material isinterbonded with a carbon reinforcing material.

There are two general methods in the prior art for the production of acarbon-carbon composite. One such method entails the chemical vapordeposition of carbon onto a structure defined by carbonaceousreinforcing material. Typically, the carbonaceous reinforcing materialcomprises carbon felt, woven carbon fibers, or the like, and the matrixis carbon deposited from the thermal pyrolysis of a carbon-bearing gas.

The other general method for producing a carbon-carbon compositecomprises fabricating a heterogeneous combination of carbonaceousreinforcing material and a pitch or an organic resin and, thereafter,subjecting this combination to a heat treatment in an inert atmosphereat a temperature of at least about 500° C. to decompose the pitch ororganic resin and thereby leave behind a carbonaceous residue bonded tothe carbonaceous reinforcing material.

The fabrication or the heterogeneous combination is accomplished byadding the pitch or organic resin to the reinforcing material at atemperature at which the pitch or resin is in a liquid state so that itcan wet the carbonaceous reinforcing material and infiltrate into andthroughout this material and act as a binder therefor.

The heat treatment to decompose the pitch or organic resin is usuallyreferred to in the art as "carbonization". The carbonaceous residuearising from the decomposed pitch or resin is sometimes referred to inthe art as a "char". When pitch is used, the carbonaceous residue isgenerally referred to as a "coke". In order to interbond the matrixmaterial with the reinforcing material, the carbon-carbon compositestructure must at least be subjected to a carbonization heat treatment.

Depending upon the particular end use of the carbon-carbon compositethat is being prepared, the carbonized composite may then be subjectedto yet an additional heat treatment step. In this further heat treatmentstep, usually referred to in the art as "graphitization", the compositeis heated to a temperature of at least 2600° C. to cause the carbonatoms in the filler and in the binder to orient into a graphite latticeconfiguration. This ordering process produces graphite with itsintermetallic properties that make it useful for many applications.

In the production of a graphite electrode, which is a specific form ofcarbon-carbon composite, coke filler particles are mixed with a pitchbinder which pitch is at a temperature such that it is in its liquidstate thereby uniformly dispersing the particulate filler and allowingthe desired article to be formed in a subsequent extrusion or moldingstep. After being formed, the "green electrode", as it is commonly knownin the art, is then subjected to a first heat treatment step which isknown as "baking" in the electrode art but which is substantiallysimilar to the "carbonization" step discussed earlier. In this step, thethermoplastic pitch binder is converted to solid coke. The graphitizedelectrode is then formed by subjecting the baked electrode to atemperature or about 2600° C. to 3000° C. for a period of about 0.5 to20 hours.

In the production or carbon-carbon composites, particularly graphiteelectrodes, one of the fundamental objectives is to obtain a compositehaving a high density and correspondingly low porosity. Relatively highporosity in a composite leads to an undesirable concomitant loss in bothstrength and other mechanical properties.

Porosity in a carbon-carbon composite generally arises as a result ofvolatilization of the low molecular weight components or either thepitch or the organic resin primarily during carbonization or baking andto some extent during graphitization. The coke yield of a pitch binderis generally about 40 to about 60 percent by weight after a heattreatment to about 500° C. at atmospheric pressure. This yield declinesslightly as the temperature is increased to about 1000° to 1400° C.Similar carbon yields (char yields) are evident with organic resinbinders. This loss of about 40 to 60 percent of the original binder as aresult of its decomposition during carbonization of the compositestructure results in voids being created producing a structure having alow density and reduced strength.

Various expedients have been employed in the prior art to avoid theformation of porosity after heat treating a combination of carbonaceousreinforcing material and a pitch or organic resin binder. One method, asdiscussed in the aforementioned "Handbook of Composites", is to applypressure to the combination of the carbonaceous reinforcing material andthe pitch binder throughout the carbonizing heat treatment in order toincrease the carbon yield.

Another method, as also discussed in the "Handbook of Composites", is toimpregnate the carbonized composite structure with the pitch or organicresin followed by an additional heat treatment, usually under pressure,to carbonize the impregnant and thereby attempt to fill any voids thatwere created by the initial carbonization step. Generally, at least twoimpregnations are used with each impregnation followed by a heattreatment under pressure.

These techniques, however, are not completely effective since voidscaused by volatilization in the binder and/or impregnant are stillcreated during the heat treatment steps.

In an effort to remedy this problem, the art has resorted to retainingthe volatile components of the pitch so as to improve both the pitchyield as well as the carbon yield by polymerizing these volatilecomponents with chemical reagents thereby forming higher molecularweight constituents which are not readily susceptible to volatilizationat carbonization temperatures.

In U.S. Pat. No. 4,096,056, for example, tar precursors are heated whileblowing an oxygen containing gas into the reactor to polymerize thevolatile components and thereby produce higher yields. In German PatentNo. 1,015,377, nitro-functional group-containing aromatic compounds areadded to pitch in order to achieve the same results by polymerizationwith these nitro-functional groups. British Patent Application No.2,045,798 also shows a process from preparing a pitch from a tar whichprocess comprises mixing the tar with a nitrating agent.

In the "Chemistry of Carbonization" and the references disclosed thereinset forth in Carbon, Vol. 20, No. 6, pp. 519-529 (1982), it is disclosedthat sulfur has also been used extensively as an additive to increasecarbon yield.

Generally, however, as the carbon yield increases, the theologicalcharacteristics of the pitch also suffer. Thus, the pitch must exhibit aviscosity which allows for its being appropriately used as a binderand/or impregnant at specific operating temperatures of variouscommercial processes. For example, in the manufacture of graphiteelectrodes, coal tar pitch binder is mixed with petroleum coke filler atabout 150° to 170° C. and extruded into a green electrode at 100° to130° C. Usually, the Mettler softening point is used as the rheologicalcriterion for pitches. As used herein, the term, "softening point"refers to the temperature at which the viscosity of the pitch is reducedto the degree required by the Mettler softening point method of ASTM D3104-75. Typical electrode pitch binders have Mettler softening pointsof about 100° to 120° C. Further polymerization can result in a pitchhaving a softening point of as much as 250° C. or more.

In addition to increasing the softening point, polymerization ofprecursor tars or pitches with a chemical reagent may also affect thegraphitizability of the resulting polymerized pitch or resin. In anarticle entitled "Chemical Changes During the Mild Air Oxidation ofPitch" by J. B. Bart and I. C. Levis, set forth in Carbon, Vol. 16, pp.439-444 (1978), it is taught that oxidation reactions may modify thestructure of the pitch by developing a network of cross-links betweenthe molecules, thereby leading to the formation of disordered carbonstructures which may prevent graphitization. In the aforementioned"Chemistry of Carbonization", it is taught that high sulfur addition mayalso result in the carbon being non-graphitizing. In U.S. Pat. No.4,066,737, which is directed to a method for making isotropic carbonfibers, it is disclosed that carbonization of highly cross-linkedmacromolecular structures containing oxygen, nitrogen or sulfur, formrigidly cross-linked aromatic planes which prevent further conversion toa graphite structure. Finally, in the Journal of Materials Science, Vol.18, pp. 3161-3176 (1983) in an article entitled "Review--Science andTechnology of Graphite Manufacture" by S. Ragan and H. Marsh, it isdisclosed that while the addition of sulfur or nitro-aromatic compoundsmay increase the carbon yield of a pitch binder upon baking, suchaddition, however, may adversely effect the graphitizability of thebinder.

The need to have good graphitizability of a binder in the production ofa carbon-carbon composite, particularly a graphite electrode, has longbeen well accepted. Good graphitizability has been correlated withdesirable electrical and thermal properties in the resulting compositestructure, particularly graphite electrodes.

In the Ragan and Harsh article, it is taught that binders used in themanufacture of electrode and graphite products need to fulfill variousspecifications. One such specification is the ability of the binder toproduce a graphitized binder coke so as to improve electrical andthermal properties. The need to produce a binder coke that can begraphitized is once again taught in a section entitled "Carbon andArtificial Graphite" of the Kirk-Othmer: Encyclopedia of ChemicalTechnology, Vol. 4, Third Edition, pp. 156-631 (1978).

The need for graphitization also carries over to the reinforcingmaterial. In an article entitled "Here is What's New in Delayed Coking"appearing in the Journal of Oil and Gas, 68:92-6 (1970) by A. Kutler, etal., it is taught that an easily graphitized coking material isdesirable for it produces the qualities of low porosity, goodconductivity and low coefficient of thermal expansion. This teaching isagain repeated in a book entitled Recent Carbon Technology edited by T.Ishikawa and T. Nagaoki (I. C. Lewis as the English editor), JEC PressInc. (1983), pp. 31-34.

A low coefficient of thermal expansion is extremely important incarbon-carbon composites, especially graphite electrodes. In addition tobeing directly indicative of the amount of thermal expansion exhibitedby the carbon-carbon composite, the coefficient of thermal expansion isalso indicative of many other properties as well. For example, bothgraphite and carbon electrodes undergo extreme thermal shock in theiruse in open-arc furnaces and submerged-arc furnaces, respectively. Anelectrode having a low coefficient of thermal expansion has a highresistance to such thermal shock. Moreover, a low coefficient of thermalexpansion is also indicative of less breakage, less consumption, as wellas low electrical resistance. In carbon-carbon composites other thanelectrodes, a low coefficient of thermal expansion may be necessary forapplications in which a close tolerance in the overall dimensions of thecomposite structure is required.

SUMMARY OF THE INVENTION

Applicants have discovered a new carbon-carbon composite structure whichavoids substantially all of the disadvantages and problems associatedwith the prior art structures discussed above.

Applicants' composite structure utilizes a particular pitch as a matrixmaterial to act as a binder and/or an impregnant. The pitch is providedin a high pitch yield, produces a high carbon yield upon carbonizationand provides increased flexural strength to the resulting carbon-carboncomposite structure. Moreover, the softening point of the pitch can alsobe controlled such that a particular softening point for a specificapplication can be obtained.

Most importantly, however, the pitch, when used in acarbon-carbon-composite structure, particularly graphite electrodes,actually improves the electrical and thermal properties thereof.

Quite surprisingly and totally unexpectedly, the specific pitch which isable to produce all of the aforementioned results is a pitch whosearomatic components have been polymerized and cross-linked to such anextent that it is poorly graphitizable or even totally non-graphitizable(i.e., forms glassy carbon). As used herein, the phrases "poorlygraphitizable pitch" or "poorly graphitizing pitch" is meant to includea pitch which may be totally non-graphitizable, and is a cross-linkedpitch, which when subjected to mesophase formation conditions, will formmesophase pitch having a domain size of less than about 30μ or will formsubstantially no mesophase pitch at all if the pitch is totallynon-graphitizable. When subjected to graphitization conditions, such across-linked pitch will form carbon layers having an inter-layer spacingof greater than about 3.38 Å as determined by X-ray diffraction.

In complete contrast to what would be predicted by one skilled in theart, based on the prior art literature discussed above, Applicants havediscovered that a carbon-carbon composite structure containing a poorlygraphitizing pitch as a binder and/or an impregnant actually reduces thecoefficient of thermal expansion of the resulting structure. Thisdiscovery was made despite the clear teachings in the prior art that apitch binder must be highly graphitizable in order to obtain goodthermal and electrical properties.

More particularly, Applicants' unitary composite structure comprises aheterogeneous combination of a carbonaceous reinforcing materialinterbonded with a matrix material, said matrix material being a poorlygraphitizing carbonaceous pitch containing aromatic components whichhave been chemically polymerized and cross-linked, wherein the structurehas a lower coefficient of thermal expansion and a higher flexuralstrength than the same structure prepared with the same pitch whosearomatic components have not been cross-linked to the same extent. Asused herein, it is to be understood that the phrase, "have not beencross-linked to the same extent" is meant to include a pitch whosearomatic components have not been cross-linked at all.

In a preferred embodiment, in order to control the softening point ofthe poorly graphitizing pitch, the pitch or its precursor is polymerizedand cross-linked to the extent that poor graphitizability is obtainedand then extracted and/or distilled to remove any low molecular weightcomponents which may have not as yet been polymerized. In anotherembodiment, the poorly graphitizing pitch may be admixed with aconventional, non-treated pitch resulting in a combined pitch having alower softening point but still possessing the characteristics desiredso as to impart improved flexural strength and a reduced coefficient ofthermal expansion to a carbon-carbon composite structure made with asuch combined pitch.

Applicants have discovered that a high degree of graphitizability,particularly in the pitch, is in fact not the panacea for obtainingdesirable properties in a carbon-carbon composite structure. Indeed,Applicants have discovered that improved flexural strength and a reducedcoefficient of thermal expansion in a carbon-carbon composite structure,particularly graphite electrodes, are obtained when the binder and/orimpregnant employed is actually poorly graphitizing, or even morepreferably, completely non-graphitizable.

Accordingly, the present invention makes it possible to obtain acarbon-carbon composite structure utilizing a pitch which provides ahigh carbon yield and density and at the same time increases theflexural strength and reduces the coefficient of thermal expansion ofsuch composite structure.

DETAILED DESCRIPTION OF THE INVENTION

The poorly graphitizing pitches used in the carbon-carbon compositestructure of the present invention are derived from either a tarprecursor or, alternatively, a pitch made from such a tar precursor.

The tars which may be employed as the starting material for preparingthese pitches include aromatic liquid oil or tar products from petroleumand coal refining. Illustrative of such tars are pyrolysis tars fromethylene processes, pyrolysis tar distillates, decant oils fromcatalytic cracking, gas oils from petroleum refining and coking, andcoal tars and coal tar distillates. Preferably, these tars shouldcontain a high concentration of aromatics which are the reactivecomponents to polymerization and cross-linking.

When the poorly graphitizing pitch is intended to be used in a processfor preparing a carbon-carbon composite structure which does not requirethe pitch to have a low softening point, then a corresponding pitch ofthe above-noted tars may be used. Generally, the polymerization andcross-linking of a commercially available pitch will result in a pitchhaving a relatively higher softening point and higher carbon yield thanthat produced from a tar precursor. Typically, a cross-linked pitch madefrom a commercially available pitch will have a softening point in therange of about 200° to 400° C. and a carbon yield of about 60 to 90%. Incontrast, a cross-linked pitch derived from a tar precursor willgenerally have a softening point in the range of about 90° to 150° C.and a carbon yield of about 40 to 65%.

The tar or pitch is treated with a cross-linking agent in accordancewith the present invention such that its aromatic components arepolymerized and cross-linked to form a disoriented, less-ordered carbonarrangement within the resulting pitch which significantly reduces itsgraphitizability. Generally, the extent of cross-linkage is indicativeof the degree of graphitizability. The more cross-linked the pitch is,the more non-graphitizable it becomes until it reaches the point that itis completely non-graphitizing and forms glassy carbon.

As used herein, the term "graphitizability" is meant to indicate thecapability of the pitch being converted thermally by heating to atemperature of about 3000° C., for a period of about 1 to 3 hours to astructure having the three-dimensional order characteristic ofpolycrystalline graphite.

The extent of graphitizability of a cross-linked pitch can be measuredby a number of different ways. One way is to determine the amount ofcross-linking agent that is bound to the aromatic components of thecross-linked pitch. The degree of graphitizability, as will be discussedmore fully below, will depend upon the amount and type of cross-linkingagent present in the pitch as well as the particular type of pitchprecursor.

In another and more preferred method, a sample of the cross-linked pitchis subjected to conditions well known in the art which ordinarily willform mesophase pitch in the sample. The degree of cross-linking isdirectly related to the mesophase domain size. Applicants havedetermined that if the domain size of mesophase pitch which does form isless than 30μ, and preferably less than about 10μ, then the treatedpitch has been polymerized and cross-linked to the extent desired. Mostpreferably, however, the treated pitch will have been cross-linked tosuch an extent that it will form mesophase pitch having a domain size ofless than about 5μ or form no mesophase pitch at all to insure thatthere will be a reduction in the coefficient of thermal expansion of aunitary composite structure made with such a cross-linked pitch.

The transformation of pitch into mesophase pitch is well known and isdiscussed in, for example, "Mesophase Microstructures in Carbonized CoalTar Pitch" by J. R. White, G. L. Guthrie, and J. O. Gardner, Carbon 5,517 (1968) and in U.S. Pat. Nos. 4,005,183, 4,032,430, 3,976,729 and4,303,631.

Generally, pitch is transformed into mesophase pitch by subjecting thepitch to a heat treatment under quiescent conditions at a temperature ofabout 350° to about 500° C. in an inert atmosphere of nitrogen, argon,xenon, helium, or the like. On heating, small insoluble liquid spheresappear in the pitch which gradually increase in size as heating iscontinued. When examined by electron diffraction and polarized lighttechniques, these spheres are shown to consist of layers of orientedmolecules aligned in the same direction. As these spheres continue togrow in size as heating is continued, they come in contact with oneanother and gradually coalesce with each other to produce large massesof aligned layers. As coalescence continues, domains of alignedmolecules much larger than the original spheres are formed. Thesedomains come together to form a bulk mesophase wherein the transitionfrom one oriented domain to another sometimes occurs smoothly andcontinuously through gradually curving lamellae and sometimes throughmore sharply curving lamellae.

The determination of the domain size in any mesophase pitch that maydevelop in the cross-linked pitch is discussed in an article entitled"Quantitative Determination of Anisotropic Domain Size in MesophasePitch" by R. T. Lewis, I. C. Lewis, R. A. Greinke, and S. L. Strongappearing in Extended Abstracts and Program, 17th Biennial Conference onCarbon, Lexington, Ky., Jun. 16-21, 1985, pg 340 and is incorporatedherein by reference including the references cited therein.

It is not necessary, however, that a separate heat treatment step becarried out on the pitch, as described above, in order to form mesophasepitch. Mesophase pitch may form simply by passing through the abovenoted temperature range as the cross-linked pitch is heated tocarbonization temperatures or higher, alone or in combination withcarbonaceous reinforcing material in a composite structure.

Cross-linked pitch which forms mesophase pitch having a domain size ofless than about 30μ will form carbon layers having an inter-layerspacing of about 3.38 Å after having been subjected to graphitizationconditions. If there is no mesophase pitch found, then the cross-linkedpitch will generally form glassy carbon upon subsequent graphitizationhaving an inter-layer spacing of about 3.40 to 3.44 Å. By comparison,perfect polycrystalline graphite has an inter-layer spacing of 3.354 Åand highly graphitizable pitch will form mesophase pitch having a domainsize of no less than about 50μ and usually about 100 to 200μ.

In yet another method for determining the extent of graphitizability ina cross-linked pitch, the pitch is actually subjected to graphitizationconditions and then, by means of X-ray diffraction analysis, theinter-layer spacing between the resulting carbon layers is measured.Such an analysis of carbonaceous materials is well known and isdiscussed in, for example, "Chemical Structure and Graphitization: X-RayDiffraction Studies of Graphites Derived From Polynuclear Aromatics",Carbon 7, 85 (1969) by T. Edstrom and I. C. Lewis, incorporated hereinby reference.

Generally, the graphitized samples are first ground such that they passthrough a 100 mesh Tyler screen. They are then mounted on a 5 ml thicktantalum fiber which has been dipped in a high temperature grease, e.g.,Apiezon grease (commercially available from Associated ElectricalIndustries, Ltd.). The function of the grease is to permit the graphiteparticles to adhere to the tantalum fiber. The use of tantalum permitsthe accurate measurement of the 002, 004, 006 and 008 reflections in thegraphite. Tantalum additionally provides distinct lines inback-reflection for photographic film shrinkage corrections.

The X-ray patterns are measured on photographic films with the use of adiffractometer, such as a Norelco diffractometer, and a powder camera,such as a Debye-Scherer powder camera. 144-6 mm diameter copperradiation and a nickel filter are employed, and the films are exposedfor a period of 20 hours. Each film is measured and corrected forshrinkage. A second correction for the thickness of the tantalum fibermount is also necessary to eliminate systematic error between resultscalculated from the 008, 006, 004, and 002 reflections.

The inter-layer spacing should be greater than about 3.38 Å andpreferably greater than about 3.40 Å in order to realize improvedflexural strength and a reduced coefficient of thermal expansion in acarbon-carbon composite made with such a cross-linked pitch as a binderand/or an impregnant. The inter-layer spacing will generally not begreater than about 3.44 Å which is recognized as being the maximumspacing for any carbon structure which has been graphitized to atemperature of 3000° C.

The tar or its corresponding pitch may be polymerized and cross-linkedusing any one or a combination of applicable cross-linking agents.

Suitable cross-linking agents include nitrating agents, elementalsulfur, oxygen, Lewis acids, halogens, strong oxidizing agents, and thelike.

Such cross-linking agents may actually interact with the aromaticcomponents of the tar or pitch and become part of the resultingpolymerized and cross-linked pitch. Alternatively, they may simply actas a catalyst for the polymerization and cross-linking reactions of thearomatic components and do not become incorporated into the resultingcross-linked pitch.

Upon treating the tar or pitch by intimate contacting with one or moreof these cross-linking agents, the tar or pitch change in character toform the poorly graphitizing pitch used in the present invention. Bycontrolling the time, temperature and the amount of cross-linking agentfor a particular tar or pitch precursor, the degree of cross-linking canbe regulated. Usually, the extent of graphitizability of a pitch is afunction of the degree of its cross-linking. If the cross-linking agentintroduces elements other than carbon into the pitch, then excessivecross-linking may result in a lower carbon yield if the carbon-carboncomposite structure containing such pitch is subsequently heated to hightemperatures such as above 1200° C. Otherwise, once the extent ofnon-graphitizability has been obtained, further cross-linking ispossible but would not be economically desirable. So too, excessivecross-linkage may also raise the softening point of the pitch to a valuewhich may not be desirable for particular applications. On the otherhand, insufficient cross-linking may result in a treated pitch which isstill highly graphitizable or, alternatively, produces a low carbonyield.

Illustrative of the nitrating agents which may be employed in theprocess of this invention are nitric acid, a mixture of nitric acid andsulfuric acid, anhydrides of nitric acid such as acetyl and benzoylnitrate, mixtures of alkali nitrates and sulfuric acid, nitric esterssuch as ethyl nitrate, N₂ O₅ or N₂ O₄ with sulfuric acid, NO₂ Cl, andthe like. When nitric acid is employed, it may be utilized in aqueoussolutions having concentrations of greater than about 20 percent byweight of nitric acid. When a mixture of aqueous solutions of nitricacid and sulfuric acid is employed, the nitric acid is present inconcentrations greater than about 25 percent by weight and the sulfuricacid is present in concentrations greater than about 30 percent byweight. Preferably, the ratio of nitric acid to sulfuric acid is 2:1 byvolume.

Typically, when using nitric acid as the nitrating agent, for example,the liquid tar precursor solid pitch is stirred with the nitric acid atroom temperature. After the desired extent of reaction has beenachieved, the solid pitch product can be separated by decantation of theacid or by simple filtration. The process may also be carried outcontinuously by spraying the tar precursor feed into a nitrating agentsolution and continuously separating the more dense product from thebottom of the reactor.

For other nitrating agents, nitration of the tar or pitch precursors maybe carried out by methods which are well known to those skilled in theart.

This cross-linking process may produce pitch yields in excess of 100%based on the weight of the starting material inasmuch as a portion ofthe nitrating agent is incorporated into the pitch that is produced. Theresulting pitch is then dried at a temperature above its melting pointin order to drive off residual water. Such drying may preferably beaccomplished with air or in a vacuum.

The extent of nitration can periodically be determined by means wellknown to those skilled in the art, e.g., by measuring the nitrogencontent of the pitch by elemental analysis or by the use of quantitativeinfrared spectroscopy. The extent of reaction should be controlled suchthat the amount of nitrogen present in the form of nitro-functionalgroups bound to the aromatic rings of the treated pitch be about 3% toabout 12% by weight and preferably about 5% to about 10% by weight.

At least about 5% by weight of nitrogen in the treated pitch is neededin order to reduce the coefficient of thermal expansion of a compositestructure made with such a cross-linked pitch. This amount of nitrogenwill generally result in the formation of glassy carbon, i.e., aninter-layer spacing of 3.40 to 3.44 Å, when the treated pitch issubjected to graphitization conditions. Although nitration beyond 12% byweight is possible and would still produce the desired results of thepresent invention, it is unnecessary to do so and economicallyundesirable.

The desired extent of nitration can be achieved by using a ratio ofabout 0.08 to 0.30 moles of nitrating agent per gram of pitch andpreferably about 0.12 to 0.20 moles of nitrating agent per gram ofpitch. When concentrated nitric acid (70%) is used as the nitratingagent, then about 10 ml of acid per gram of pitch and a reaction time of3 hours at 25° C. will usually result in a cross-linked pitch containingabout 5.5% nitrogen. Treatment for longer periods of time, up to 6hours, will generally increase the nitrogen content to about 10%.

When using elemental sulfur as a cross-linking agent, the tar or pitchis mixed with sulfur in a ratio of about 0.0015 to 0.0075 moles ofsulfur per gram of tar (pitch) and more preferably about 0.003 to 0.005moles of sulfur per gram of tar (pitch). This mixture is then heated ata temperature of about 150° to 350° C. for about 1 to 5 hours. Thesulfur/tar (pitch) ratio controls the extent of reaction and the degreeof graphitizability.

In order to achieve the desired degree of poor graphitizability,applicants have determined that about 1.3% to about 12% by weight ofsulfur should be present and cross-linked to the aromatic rings of theresulting polymerized and cross-linked pitch, the amount varyingdepending upon the particular precursor being treated.

With a sulfur content of about 1.3 to 5% by weight, the cross-linkedpitch will generally form a mesophase pitch having a domain size ofabout 5 to 30μ resulting in a composite structure made with such across-linked pitch to have increased flexural strength as compared to asimilar structure made with conventional, untreated pitch. If the sulfurcontent is greater than about 5% by weight, then the cross-linked pitchwill usually form no mesophase at all (i.e., is totallynon-graphitizable and forms glassy carbon upon graphitization) and willalso reduce the coefficient of thermal expansion of a compositestructure made with such a pitch.

When polymerizing and cross-linking the tar or pitch with oxygen, air,ozone or a combination thereof is bubbled through the tar or moltenpitch, preferably while stirring at a temperature of about 200° to 350°C. and preferably about 250° to 300° C. for a period of about 1 to 10hours and preferably about 1 to 4 hours, depending upon the specificstarting material. While temperatures below 200° C. may be used, if sodesired, the oxidation reaction proceeds at a very slow rate at such lowtemperatures. Above 350° C., excessive distillation of the volatilecomponents may occur and there is even a danger of combustion. Theparameters of temperature and time are interrelated so that the sameextent of oxidation can be achieved with a high temperature and shorttime or, alternatively, low temperature and longer time.

Generally, air or oxygen is introduced into the tar or pitch startingmaterial at a rate of about 0.0014 to 0.06 standard cubic meters perhour (SCMH) or more for about 50 grams of the starting material.

Preferably, the resulting polymerized and cross-linked, poorlygraphitizing pitch prepared using oxygen as a cross-linking agent hasabout 2% to about 12% by weight of oxygen bound to the aromatic rings ofthe pitch and more preferably greater than about 4% by weight in orderto obtain the degree of poor graphitizability which applicants havedetermined produce the desired results of the present invention.

Lewis acids are capable of inducing aromatic polymerization andcross-linking in the tar and/or pitch starting materials. The Lewisacids themselves are not incorporated into the final cross-linked pitch.

Aside from AlCl₃, which will not work, all of the conventionallyacceptable Lewis acids are applicable in the present invention andinclude metal halides such as FeCl₃, SnCl₂, SnCl₄, FeBr₃, ZnCl₂, SbCl₃,SbCl₅, CoCl₂, BF₃, combinations thereof, and the like. Other Lewisacids, which are well-known in the art, can also be employed.

In preparing the poorly graphitizing pitch, the Lewis acid is admixedwith the tar or pitch in a ratio of 0.0005 to 0.006 moles of Lewis acidper gram tar (pitch) and preferably about 0.002 to 0.004 moles of Lewisacid per gram of tar or pitch to help insure the reduction of thecoefficient of thermal expansion in a carbon-carbon composite using sucha cross-linked pitch. The mixture is then heated at a temperature offrom about 100° to 300° C. with stirring for a period of about 1 to 6hours. A non-reactive solvent with a boiling point of at least 100° C.or more, such as, chlorobenzene, trichlorobenzene, nitrobenzene, etc.,can be employed with the Lewis acid if it is a solid. On the other hand,if the Lewis acid is a liquid, for example, SbCl₅, then the pitch or tarcan be stirred directly with such a Lewis acid. With very reactive Lewisacids such as SbCl₅, the polymerization and cross-linking can beachieved at a temperature below 100° C., preferably near roomtemperature.

In a preferred embodiment, the Lewis acid can be admixed with an alkalimetal halide such as KCl, NaCl, KBr, CsCl, etc., to form a low meltingeutectic where this eutectic can serve as the liquid reaction medium.

After the reaction has been carried out to the extent desired, it ispreferable to remove the components of the Lewis acid inasmuch as theymay be characterized as impurities and will not contribute to the carbonyield of the resulting pitch. Accordingly, the reacted mixture istreated with water and if desired, water with hydrochloric acid, toremove the Lewis acid components which are soluble therein. Afterseparation of the product by filtration, the pitch product is dried invacuum.

Other cross-linking agents which are applicable herein include halogenssuch as chlorine, bromine, iodine, and the like, as well as strongoxidizing agents such as peroxide, permanganates, persulfates, chloratesalts, and the like. Still additional cross-linking agents can readilybe determined by those skilled in the art.

In a preferred embodiment, after the tar or pitch has been polymerizedand cross-linked to the extent necessary to provide the poorgraphitizability required by the present invention, the carbon yield ofthis treated poorly graphitizing pitch can further be improved bysubjecting the cross-linked pitch to a subsequent distillation and/orextraction step.

In the distillation step, a high vacuum, generally about less than 10 mmof mercury, and temperatures of 350° C. or below (in order to avoid theformation of mesophase) are employed.

The cross-linked pitch is distilled with agitation which may be achievedby introducing an inert gas through the molten pitch.

In the alternative extraction process, a series of solvents with varyingdegrees of dissolving capability is used. The cross-linked pitch issequentially dissolved in various solvents thereby removing lowmolecular weight components which have not polymerized. Solvents whichare applicable for such an extraction step include, but are not limitedto, methyl alcohol/petroleum ether, petroleum ether, petroleumether/toluene, toluene, and the like. Such solvents are well known tothose skilled in the art and other applicable solvents can equally beemployed.

In still another embodiment of the present invention, a cross-linked,poorly graphitizing pitch prepared in accordance with the presentinvention which may possess a softening point which is higher than thatdesired may be admixed with an untreated pitch having a low softeningpoint such that the resulting mixture provides a pitch which is stillsubstantially poorly graphitizing but which, now has a reduced softeningpoint. Generally, when so admixing a cross-linked pitch with anuntreated pitch, the amount of untreated pitch present in the resultingmixture should be no more that about 90% by weight and preferably nomore than about 75% by weight in order to retain the poorly graphitizingcharacteristic of the resulting mixed pitch.

As an example, a cross-linked pitch made by air oxidation having asoftening point of 200° C. can be admixed with a coal tar pitch having asoftening point of 110° C. to result in a mixture containing 60% byweight of the untreated pitch having a softening point of 160° C. whichpitch still possesses the required poor graphitizability to provideincreased flexural strength and reduced coefficient of thermal expansionin a composite structure using this pitch admixture.

The preparation of such admixed pitch is especially desirable whenemploying a poorly graphitizing, nitrated pitch. Generally, a nitratedpitch undergoes a highly exothermic reaction during carbonization. Inorder to control this reaction, the nitrated pitch is admixed with anuntreated pitch wherein the resulting heat of reaction is reduced uponsubsequent carbonization.

Even when only 12.5% by weight of a nitrated pitch having a softeningpoint of 100° C. is admixed with a coal tar pitch having a softeningpoint of 110° C., the resulting admixture still results in a poorlygraphitizing pitch which provides the desired properties of the presentinvention.

Once the cross-linked pitch having the necessary degree of poorgraphitizability is provided, it is ready to be combined with thecarbonaceous reinforcing material to form the ultimate carbon-carboncomposite.

The carbonaceous reinforcing materials that are applicable in thepresent invention are substantially thermoplastically non-deformable fortemperatures less than 1000° C. and include carbon or graphite fibers,petroleum coke, natural graphite, carbon black, anthracite, and thelike.

The fibers may be derived from various precursor materials such asisotropic pitch, mesophase pitch, polyacrylonitrile, etc. Suitablefibers include commercially available fibers which generally have anaverage diameter in the range of from about 5 to about 15μ.

The fibers used for the carbonaceous reinforcing material may be woven,non-woven, knitted, structured, wound on a form such as a mandrel or thelike, or aligned. In addition, the fibers may be used as "choppedfibers" to fill the matrix material. As used in the art, "choppedfibers" are short links of carbon or graphite fibers usually having anaverage length of about 1 to about 8 mm. Such chopped fibers areproduced by methods well known in the art.

Carbon particles such as petroleum coke, natural graphite, etc., can beused alone as the reinforcing material such as in the manufacture ofgraphite electrodes. Alternatively, such carbon particles may be used inconjunction with the carbon or graphite fibers as a filler material. Thepoorly graphitizing matrix pitch material is combined with thecarbonaceous reinforcing material in any conventional manner which iswell known to those skilled in the art.

Depending upon the particular end use of the unitary, carbon-carboncomposite structure being prepared, chopped graphite or carbon fibersmay, for example, simply be mixed with the matrix pitch material whilein its molten state and then compression molded or extruded into adesired shape or form. Alternatively, a woven or knitted fabric ornon-woven fabric of carbon fibers may be shaped by means well known inthe art and the matrix pitch material is then applied to the shapedreinforcing material in any suitable manner such as by spraying,dipping, and the like. Any conventional method for preparing thecomposite structure may be employed such as those disclosed in U.S. Pat.No. 4,131,708, which is incorporated herein by reference.

In the preparation of a graphite electrode, a calcined coke filler,derived from either petroleum or coal, is mixed with molten matrixmaterial and is then formed into the desired shape by either extrusionor molding.

Where the carbonaceous reinforcing material does not have sufficientintegrity of its own, such as chopped fibers, the initial function ofthe matrix pitch material is to act as a binder and uniformly dispersesuch a filler and allow the desired article to be formed. In thefinished carbon-carbon composite product, the binder acts as the meansby which the fibers are interbonded to one another and to the matrixmaterial. In the case of a reinforcing material which has structuralintegrity, such as a woven fabric comprised of carbon fibers, the binderassists in allowing the desired article to be formed and also interbondswith such reinforcing material in the resulting product.

After being formed, the combination of reinforcing material and matrixmaterial is then subjected to carbonization conditions so as to form theinterbonded, unitary composite structure. As a result of thiscarbonization, the matrix pitch material is converted to an essentiallynon-thermoplastically deformable state.

Usually, carbonization is effected at a temperature of about 900° C. toabout 1500° C., preferably from about 1000° C. to 1400° C. Generally,residence times are from about 0.5 minute to about 3 hours, preferablyfrom about 1 minute to about 1 hour. While more extended heating timescan be employed, such residence times are uneconomical and, as apractical matter, there is no advantage in employing such long periods.Such a carbonization step is well known in the art.

After the initial carbonization step, it may be desirable to addadditional matrix pitch material to the unitary, carbon-carbon compositestructure, this time to act as an impregnant and fill any voids whichmay have been created during the carbonization step so as to increasethe density and correspondingly increase the strength of the resultingproduct. After additional matrix material has been added to thecomposite structure, the structure is once again subjected tocarbonization conditions. These steps may be repeated again and again asdesired.

If desired, the resulting carbon-carbon composites may then begraphitized to produce articles such as electrodes, structural carbonsand graphites, and the like, for various electro-mechanicalapplications. Graphitization conditions are well known to those skilledin the art and generally include subjecting the composite to atemperature of from about 2600° C. to about 3000° C. for a time periodfrom about 0.5 to 20 hours. With well graphitizing filler materials,this heat treatment will convert the carbonaceous reinforcing materialin the composite structure to polycrystalline graphite. However, thecross-linked matrix pitch material, which is only poorly graphitizing,will form carbon layers having an inter-layer spacing of greater thanabout 3.38μ.

The unitary composite structure comprising a heterogeneous combinationof carbonaceous reinforcing material interbonded with a matrix materialwhich is a poorly graphitizing, cross-linked carbonaceous pitch has ahigher flexural strength when compared to the same structure prepared inthe same way using the same constituents with the only exception beingthat the pitch is one which has not been cross-linked at all orcross-linked to the extent required by the present invention. Indeed,even if the pitch which is not cross-linked is treated by physicallydistilling or extracting it such that volatiles are removed and itscarbon yield increased to the point that it is equal to the carbon yieldof the cross-linked pitch of the present invention, the flexuralstrength of a composite structure made with such a physically treatedpitch would generally still be less than the same structure made withthe poorly graphitizing pitch. This is totally unexpected for it hasbeen generally accepted in this art that pitches having the same carbonyield will usually provide composites of comparable flexural strengths.Applicants have found, however, that even if the carbon yields of thepoorly graphitizing pitch and conventional pitch are the same, theflexural strength of a composite made with the poorly graphitizing pitchwill generally be substantially higher.

Provided that the poorly graphitizing pitch has been cross-linked to theextent that it forms mesophase pitch having a domain size of less than5μ or forms substantially no mesophase pitch (where the absence ofmesophase pitch leads to the formation of glassy carbon upon beingsubjected to graphitization conditions), a composite structure made withsuch a cross-linked pitch will have a lower coefficient of thermalexpansion than the same structure prepared in the same way with the onlyexception being the use of a pitch which has not been cross-linked tothe extent required by the present invention.

Composite structures prepared in accordance with the present inventionwill have a flexural strength that is at least about 20 to 40% greaterthan composite structures made with a conventional, highly graphitizablepitch matrix, even if it has the same carbon yield. Similarly, thereduction in the coefficient of thermal expansion can be as much asabout 0.2×10⁻⁶ /°C. as compared to a composite structure made with agraphitizable pitch, regardless of its carbon yield.

In addition to improved flexural strength and a reduced coefficient ofthermal expansion, the composite structures prepared in accordance withthe present invention also have increased carbon yield, increaseddensity and overall improvement in their thermal and electricalproperties.

Having described the basic concepts of the this invention, the followingExamples are set forth to illustrate the same. They are not, however, tobe construed as limiting the invention in any manner.

Flexural strength measurements are carried out pursuant to ASTM MethodC-651.

Coefficient of thermal expansion measurements are carried out onextruded graphite rods by means of the Lamb's roller technique, which iswell known in the art. This method measures the difference in thermalexpansion between two dissimilar materials. Generally, the test specimenis mounted in a frame parallel to a bar, known as the "unispan" bar,made of a different material. The teat specimen and unispan bar arefirmly fixed to each other and the frame at one end, the other end ofeach being free. A rod attached to a mirror is placed between the testspecimen and unispan bar near their free ends, in contact with each. Areticle and scale are mounted at a fixed distance from the expansionunit and are able to measure rotation of the mirror in response to thedifferential expansion of the unispan bar and the test specimen.

A precisely calibrated lampblack-based standard rod is mounted in placeof the test specimen. The entire frame-unispan bar-lampblack rodassembly is heated through the temperature range of 30° C. to 100° C. Asthe assembly is heated through this range, the rod and mirror rotate dueto the differential expansion of the unispan bar and lampblack rod. Thedifference in degree of rotation between 30° C. and 100° C. is measured,and from this the coefficient of thermal expansion of the bar isdetermined. The process is repeated with the test specimen in place ofthe lampblack rod and with the unispan bar serving as the standard. Thecoefficient of thermal expansion of the test specimen is then determinedby comparison with the unispan bar.

EXAMPLE 1

10 grams of a decant oil obtained from the catalytic cracking of gasoil, having an aromatic proton content of 37%, is stirred in a 400 mlbeaker using a magnetic stirrer. 100 ml of 70% nitric acid aqueoussolution is poured into the beaker and the mixture is then stirred at25° C. for 3 hours.

The reacted mixture is filtered by being passed through a Buchner funnelwhich is maintained under a vacuum by the use of a water aspirator. Thesolid residue consisting of the nitrated pitch product remaining afterfiltration is then washed with about 50 ml of water and dried in avacuum oven at 50° C. A solid nitrated pitch in a yield of 118%, basedon the weight of the original oil, is obtained.

The polymerized and cross-linked pitch contains 5.4% by weight ofnitrogen bound in the form of nitro-functional groups to the aromaticcomponents of the pitch. Additionally, the pitch has a carbon yield (asmeasured by the modified Conradson carbon content test as described onpage 521, Volume II of Analytical Methods for Coal and Coal Products, C.Carr, Jr., Academic Press (1978)) of 60% and a softening point (MettlerSoftening Point) of 100° C.

The extent of graphitizability of this pitch is determined by subjectinga sample of the pitch to mesophase forming conditions. Approximately 2grams of the pitch is placed in a porcelain container having dimensionsof 12×12×75 mm. Aluminum foil is placed loosely over the top of thecontainer to help reduce the escape of volatiles to minimize foaming. Analuminum holder surrounds the container for uniform heating. A metalsheathed thermocouple connected to the holder is used for measuring thetemperature. The entire assembly is then inserted into a 3.5 cm diameterquartz tube which in turn is inserted into a 4 cm diameter tube furnace.The quartz tube is closed at both ends except for the provision of a 5mm opening at each end to accommodate the introduction and release of aninert gas by means of a tube and access to the thermocouple. Nitrogen isintroduced into the quartz tube at a flow rate of 0.0014 standard cubicmeters per hour during the entire heat treatment. The tube furnace isthen heated to a temperature of 400° C. and maintained at thattemperature for 12 hours.

After the sample is allowed to cool to room temperature, it is thenexamined by polarized light microscopy. A specimen for microscopy isprepared by encapsulating pieces of the pitch in a ring mold with aliquid epoxy which is cured at room temperature. The specimen is,thenpolished by grinding with a Texmet (A. B. Buehler Corp.) disc chargedwith 3 μm sized alumina particles.

Inspection of the thusly treated sample by polarized light microscopyusing a Bausch and Lomb metallographic polarized light microscopereveals that it is completely isotropic without the presence of anymesophase pitch. Consequently, it is determined that the decant oil hasbeen polymerized and cross-linked with the nitrating agent to thedesired extent of poor graphitizability.

The solid nitrated pitch is then blended with standard, solidcommercially available coal tar pitch obtained from physicaldistillation of coal tar, having a softening point of 110° C. and acarbon yield of 60%, and a quinoline insoluble content of 12% by weightin a weight ratio of 1:1 using a 400 ml beaker and stirred with a glassstirring rod. The resulting admixture has a softening point of 100° C.and a carbon yield of 62%.

The blended pitch is then used as a binder to extrude a 19 mm diametergreen electrode. The blended pitch matrix material is admixed withcalcined petroleum coke in a ratio of 30 parts pitch to 100 parts cokeby placing the coke into a hot oil jacketed 4 liter mixer preheated to160° C. and then adding the blended pitch thereto. The admixture of cokeand pitch is then mechanically stirred at a temperature of 160° C. forabout 60 minutes. The admixture is then allowed to cool to about 100° C.and placed into a horizontal press-type extruder having a 19 mm diameterbore equipped with electrical heating elements. The extruder isevacuated and maintained at 120° C. The admixture is extruded from theextruder at a pressure of 21 kg/cm² to form a cylindrical rod 19 mm indiameter and 14 cm long. The rod is then baked by placing it in a saggerpacked with calcined coke and heated at a rate of 3° C./hr to atemperature of 500° C., in an electrically heated furnace and thencarbonized by heating at a rate of 10° C./hr to a temperature of 950°C., and finally graphitized by heating at a rate of 200° C./hr to atemperature of 3000° C. and held at 3000° C. for 2 hours in a hightemperature electrically heated furnace.

After carbonization, a portion of the composite structure comprising thepitch binder and coke filler mix is examined by optical microscopy usingpolarized light and it is noted that the nitrated pitch portion of thestructure produced a non-graphitized, glassy carbon which shows noanisotropy.

The characteristics of the graphitized structure for two separate trialruns are set forth in Table I below:

                  TABLE I                                                         ______________________________________                                        Properties of Graphite Electrode Made                                         With 50% Nitrated Pitch/50% Coal                                              Tar Pitch Binder                                                                                        Coeff. of                                                            Flexural Thermal                                                    Density   Strength Expansion                                                  (g/cc)    (kgs/cm.sup.2)                                                                         (×10.sup.-6 /°C.)                      ______________________________________                                        Run 1a   1.58        128.88   0.23                                            Run 1b   1.57        125.43   0.25                                            ______________________________________                                    

EXAMPLE 2 (Comparison Example)

For comparison, another graphite electrode is prepared in accordancewith the procedure set forth in Example 1 using the exact same cokefiller with the exception being that in this comparison example, thebinder pitch consists entirely of the commercially available coal tarpitch which is used to make the blended pitch in Example 1. The nitratedpitch made in Example 1 is not used at all in this comparison example.

After carbonization of the green electrode, optical microscopy usingpolarized light on a portion of the carbonized electrode reveals thatthe carbonized coal tar pitch is highly graphitizable and exhibits largeanisotropic domains.

The properties of the graphitized electrode are set forth in Table IIbelow:

                  TABLE II                                                        ______________________________________                                        Properties of Graphite Electrode Made                                         With 100% Coal Tar Pitch Binder                                                                      Coeff of                                                             Flexural Thermal                                                Density       Strength Expansion                                              (g/cc)        (kgs/cm.sup.2)                                                                         (×10.sup.-6 /°C.)                         ______________________________________                                        1.53          102.86   0.42                                                   ______________________________________                                    

As is readily apparent, the graphite electrode of Example 1 made withthe non-graphitizable pitch has a higher flexural strength and a reducedcoefficient of thermal expansion than the graphite electrode made inthis Example which is made from a conventional pitch binder. So too, thedensity in the graphite electrode containing the nitrated pitch wasdesirably greater.

EXAMPLE 3

Coal tar is polymerized and cross-linked with elemental sulfur in anumber of different trials to form poorly graphitizing matrix pitchmaterial. In the trials, the weight ratio of the tar to sulfur is variedin conjunction with the reaction time and temperature. Coal tar isplaced in a 500 ml Resin flask to which the appropriate amount of sulfuris added. The flask is fitted with a reflux condenser, thermocouple, anda mechanical stirrer. The mixture is then heated with a heating mantlewhile stirring to the appropriate temperature and maintained at thattemperature for the appropriate time. After allowing the reacted mixtureto cool to room temperature, it is weighed and portions thereofsubjected to various characterization tests. The mesophase domain sizeis determined by heating the pitch samples to 400° C. for 12 hours inthe manner described in Example 1.

The results of these trials are set forth in Table III as follows:

                                      TABLE III                                   __________________________________________________________________________    Coal Tar Cross-Linked With Sulfur                                                                                           Mesophase                       Trial                                                                             Tar/S                                                                             Temp.                                                                              Time                                                                              Pitch Yield                                                                          Softening Point                                                                       Carbon Yield                                                                          S in Pitch                                                                          Domain Size                     #   Ratio                                                                             (°C.)                                                                       (hrs)                                                                             (%)    (°C.)                                                                          (%)     (%)   (microns)                       __________________________________________________________________________    3a  10/1                                                                              230  1.0 82     96.5    49.8    4.5   1-5                             3b  10/1                                                                              250  1.0 79     116.3   52.8    4.2   1-5                             3c   6/1                                                                              230  1.0 82     130.4   58.9    9.5   No mesophase                                                                  (forms glassy carbon)           3d   5/1                                                                              220  1.0 89     94.2    55.4    12.6  No mesophase                                                                  (forms glassy carbon)           3e   5/1                                                                              230  1.0 82     144.0   61.0    12.0  No mesophase                                                                  (forms glassy                   __________________________________________________________________________                                                  carbon)                     

As is seen, the more sulfur that is mixed with the tar precursor, themore non-graphitizable is the resulting pitch. However, excessive sulfurmay detrimentally affect the carbon yield and increase the softeningpoint beyond that which may be commercially acceptable for a particularapplication.

EXAMPLE 4

Example 3 is repeated with the exception that instead of coal tar,pyrolysis tar is used instead. The results of these trials are set forthin Table IV below:

                                      TABLE IV                                    __________________________________________________________________________    Pyrolysis Tar Cross-Linked With Sulfur                                                                                      Mesophase                       Trial                                                                             Tar/S                                                                             Temp.                                                                              Time                                                                              Pitch Yield                                                                          Softening Point                                                                       Carbon Yield                                                                          S in Pitch                                                                          Domain Size                     #   Ratio                                                                             (°C.)                                                                       (hrs)                                                                             (%)    (°C.)                                                                          (%)     (%)   (microns)                       __________________________________________________________________________    4a  10/1                                                                              220  1.0 88     90.1    35.0    1.2   1-5                             4b  10/1                                                                              250  1.0 85     91.6    36.0    1.3   1-5                             4c  10/1                                                                              300  1.0 77     96.3    41.0    1.5   1-5                             4d  7.5/1                                                                             250  1.0 82     118.2   40.8    3.1   1-5                             4e   5/1                                                                              200  1.5 87     105.4   44.9    10.4  No mesophase                                                                  (forms glassy carbon)           4f   5/1                                                                              220  1.0 81     168.3   48.6    6.5   No mesophase                                                                  (forms glassy                   __________________________________________________________________________                                                  carbon)                     

EXAMPLE 5

Various commercially available pitches are polymerized and cross-linkedwith elemental sulfur at various weight ratios, reaction temperaturesand residence times to produce high softening point pitches in themanner described in Example 3. The commercially available pitchestreated include a coal tar pitch with a softening point of 108° C.containing a quinoline insoluble content of 15% and forms a mesophasepitch having a domain size of about 50-60 μ; a coal tar pitch with asoftening point of 132° C. having no quinoline insoluble content andwhich forms a mesophase pitch having a domain size of about 150 to 200μ;and a petroleum pitch having no quinoline insolubles which forms amesophase pitch having a domain size of about 150 to 200μ. The resultsof these trials are set forth in Table V below:

                                      TABLE V                                     __________________________________________________________________________    Pitches Cross-Linked With Sulfur                                                                                                  Mesophase                 Trial       Pitch/S                                                                           Temp.                                                                              Time                                                                              Pitch Yield                                                                          Soft Point                                                                          Carbon Yield                                                                          S in Pitch                                                                          Domain Size               #  Type of Pitch                                                                          Ratio                                                                             (°C.)                                                                       (hrs)                                                                             (%)    (°C.)                                                                        (%)     (%)   (microns)                 __________________________________________________________________________    5a Coal Tar 10/1                                                                              220  1.0 94     188   69.2    4.8   1-5                          (S.P. = 108° C.)                                                    5b Coal Tar 10/1                                                                              300  1.0 81     275   75.0    5.0   1-5                          (S.P. = 108° C.)                                                    5c Coal Tar 20/3                                                                              250  0.3 91     263   76.3    6.8   no mesophase                 (S.P. = 108° C.)                          (forms glassy                                                                 carbon)                   5d Coal Tar  4/1                                                                              220  1.0 93     191   77.5    15.1  no mesophase                 (S.P. = 108° C.)                          (forms glassy                                                                 carbon)                   5e Coal Tar 10/1                                                                              220  1.0 95     186   68.4    5.94  1-5                          (S.P. = 132° C.)                                                    5f Coal Tar 20/3                                                                              293  0.1 90     306   76.0    5.5   1-5                          (S.P. = 132° C.)                                                    5g Coal Tar  4/1                                                                              220  1.0 94     181   76.6    16.8  no mesophase                 (S.P. = 132° C.)                          (forms glassy                                                                 carbon)                   5h Petroleum                                                                              10/1                                                                              220  0.3 96     214   71.6    6.0   1-5                          (S.P. = 120° C.)                                                    5i Petroleum                                                                               4/1                                                                              220  0.7 93     227   81.4    15.9  no mesophase                 (S.P. = 120° C.)                          (forms glassy             __________________________________________________________________________                                                        carbon)               

EXAMPLE 6

375 grams of a coal tar is heated with 37.5 grams of elemental sulfur at250° C. for 2 hours with stirring in the same manner as described inExample 3. The resulting polymerized and cross-linked pitch product hasan 82% pitch yield, based on the weight of the original coal tar, andhas a softening point of 110° C. and a carbon yield of 49.8%.

A portion of the pitch is then heated to 400° C. for 12 hours to formmesophase in the same manner as described in Example 1. An examinationof the mesophase formed shows that it has domains of about 2 microns insize thereby indicating the desired extent of non-graphitizability.

The matrix pitch material is then used as a binder to form a graphiteelectrode in the same manner as described in Example 1. The pitch isadmixed with petroleum coke at a level of 30 parts pitch per 100 partscoke, extruded into the shape of a cylindrical rod, baked by heating ata rate of 3° C./hr to a temperature of 500° C., 10° C./hr to 950° C.,then graphitized at a temperature of 3000° C. for 2 hours.

The graphitized rod has a coefficient of thermal expansion of 0.39×10⁻⁶/°C.

EXAMPLE 6 (Comparison Example)

The graphitized rod of Example 5 is prepared once again, with the onlyexception that the pitch used as a binder is a conventional coal tarpitch having a softening point of 100° C. The coefficient of thermalexpansion of this comparative graphite electrode is 0.45×10⁻⁶ /°C., anincrease of 15.4%.

EXAMPLE 7

A coal tar distillate is polymerized and cross-linked by air oxidationby placing the coal tar distillate in a 500 ml Resin flask fitted withan air inlet tube, a mechanical stirrer, a distillation unit, and athermocouple. Air is bubbled through the tar in an amount of 0.06 SCMHper 100 grams of tar while stirring and heating with a heating mantle toa temperature of 250° C. for a period of 4 hours.

A cross-linked, poorly graphitizing pitch is obtained having a softeningpoint of 213° C. and a carbon yield of 60%.

This solid oxidized pitch is blended with a standard commerciallyavailable coal tar pitch which has a softening point of 110° C. and acarbon yield 60% in a weight ratio of 60% air blown pitch/40% standardpitch by mechanically stirring a melted mixture of the oxidized pitchand commercial pitch in a 400 ml aluminum container heated with aheating mantle at a temperature of 285° C. for 1/2 hour under a nitrogenblanket. The resulting blend has a softening point of 165° C. and acarbon yield of 60%.

A 19 mm graphite electrode is formed using the pitch blend as a matrixbinder in accordance with the procedure described in Example 1 and isdetermined to have the characteristics set forth in Table VI below:

                  TABLE VI                                                        ______________________________________                                        Properties of Graphite Electrode                                              Made With 60% Air-Blown Pitch/                                                40% Coal Tar Pitch Binder                                                                            Coeff of                                                             Flexural Thermal                                                Density       Strength Expansion                                              (g/cc)        (kgs/cm.sup.2)                                                                         (×10.sup.-6 /°C.)                         ______________________________________                                        1.55          116.50   0.26                                                   ______________________________________                                         S.P. = 165° C.                                                         MCC = 60%                                                                

EXAMPLE 8 (Comparison Example)

Example 7 is repeated with the only exception that instead of using theblended pitch, the matrix binder material is a standard coal tar pitchhaving a softening point of 110° C. and a carbon yield of 60%. After thegraphite electrode is prepared using this coal tar pitch as a binder inaccordance with the procedure described in Example 1, the followingproperties are obtained as set forth in Table VII below:

                  TABLE VII                                                       ______________________________________                                        Properties of Graphite Electrode Made                                         With 100% Coal Tar Pitch Binder                                                                      Coeff. of                                                            Flexural Thermal                                                Density       Strength Expansion                                              (g/cc)        (kgs/cm.sup.2)                                                                         (×10.sup.-6 /°C.)                         ______________________________________                                        1.54          105.89   0.40                                                   ______________________________________                                         S.P. = 110° C.                                                         M.C.C. = 60%                                                             

When comparing the graphite electrode prepared in Example 7 which ismade in accordance with the present invention and the electrode made inthis Example which has been made in accordance with the prior art, theflexural strength of the electrode of Example 7is increased whereas thecoefficient of thermal expansion is desirably reduced.

EXAMPLE 9

Various precursors are treated with Lewis acids so as to formpolymerized and cross-linked, poorly graphitizing pitches. Theprecursors are cross-linked by adding the appropriate amount of LewisAcid to the appropriate amount of precursor in a 100 ml flask fittedwith a reflux condenser, a thermometer, and a magnetic stirrer. Themixture is heated with a heating mantle while stirring to theappropriate temperature and for the appropriate time as indicated inTable VIII below. After cooling to room temperature, the mixture isadded to 100 ml of concentrated hydrochloric acid contained in a 400 mlbeaker and stirred with a magnetic stirrer for 1/2 hour. The mixture isthen filtered through a Buchner funnel evacuated with a water aspirator.The solid, cross-linked pitch is then washed with 50 ml of water andre-filtered. The pitch is then dried in a vacuum oven at 50° C. for 1/2hour. The conditions and results of the various trials are set forth inTable VIII as follows:

                  TABLE VIII                                                      ______________________________________                                        Pitches Made With Lewis Acids                                                 as Cross-Linking Agents                                                                                     React.                                                                              React.                                                                              Pitch                               Trial             Cross-Linking                                                                             Temp. Time  Yield                               #    Precursor    Agent       (°C.)                                                                        (hrs.)                                                                              (%)                                 ______________________________________                                        11 a Decant Oil (20 g)                                                                          SnCl.sub.4 (10 g)                                                                         110   2     95                                  11 b Pyrolysis Tar                                                                              ZnCl.sub.2 (13.8 g)                                                                       120   3     56                                       (13.8 g)     +                                                                             Acetic Acid                                                                   (50 ml)                                                     11 c Pyrolysis Tar (10 g)                                                                       CuCl.sub.2 (10 g)                                                                         350   2     100                                       distilled to remove                                                                       +                                                                370° C. distillate!                                                                 ZnCl.sub.2 (10 g)                                           11 d Pyrolysis Tar (5 g)                                                                        SbCl.sub.3 (10 g)                                                                         150   3     100                                       distilled to remove                                                                       +                                                                370° C. distillate!                                                                 NaCl (5 g)                                                  11 e Pyrolysis Tar (5 g)                                                                        SbCl.sub.5 (10 g)                                                                         25    1     100                                       distilled to remove                                                          370° C. distillate!                                               11 f Pyrolysis Tar (5 g)                                                                        I.sub.2 (2 g)                                                                             160   2     100                                       distilled to remove                                                          370° C. distillate!                                               11 g Pyrolysis Tar                                                                              FeCl.sub.3 (10 g)                                                                         310   1.5   90                                       Residue (10 g)                                                           ______________________________________                                    

All of the cross-linked pitches are then subjected to conditions whichfavor the production of mesophase, particularly, heating these pitchesat a temperature of 400° C. for 12 hours using the procedure describedin Example 1. Polarized light microscopy shows that the pitches producedno mesophase at all or, alternatively, produced a mesophase having adomain size which is less than 20 m indicating that the resulting pitchis indeed cross-linked to the extent necessary to providenon-graphitizability.

EXAMPLE 10

A light coal tar distillate is heated for 2 hours at a temperature of250° C. and then for 1 hour at a temperature of 275° C. while air isbubbled through the distillate at a flow rate of 0.06 SCMH per 50 gramsof distillate using the procedure described in Example 7. A solid pitchproduct is obtained in a yield of 37%, based on the weight of theoriginal tar distillate, a carbon yield of 38% and a softening point of83° C.

A portion of the pitch is then subjected to mesophase forming conditionsby heating the pitch at 400° C. for 12 hours in the manner described inExample 1. Examination by polarized light microscopy reveals that thepitch forms no mesophase and will therefore form glassy carbon whensubjected to subsequent graphitization conditions.

The pitch is then distilled by placing the pitch in a ceramic containerand heating the pitch under a nitrogen blanket at a temperature of 350°C. at atmospheric pressure in a tube furnace for a period of 1/2 hour.

The distilled pitch has a softening point of 297° C. and a carbon yieldof 63%, a gain of about 65.7% in carbon yield as a result of thedistillation step.

EXAMPLE 11

A coal tar distillate is polymerized and cross-linked by air oxidationby bubbling air through the distillate in an amount of 0.06 SCMH per 100grams of distillate at a temperature of 250° C. for a time period of 4hours in the same manner as described in Example 7. A poorlygraphitizing, cross-linked pitch is obtained having a softening point of213° C. which forms mesophase pitch having a domain size of about 1 to10μ.

This pitch is impregnated into small specimens of a porous carbonfiber/carbon binder preform. The carbon preform specimens were obtainedfrom a hollow cylinder of approximately 1.27 cm wall thickness,fabricated according to the following process:

a) Union Carbide Grade WCA graphite cloth is preimpregnated with acarbon-black filled phenolic resin, then heated to a tacky state.

b) The prereg is cut into thin strips which are laid in a "rosettepattern" over a cylindrical male mandrel. The strips are laid such thattheir longitudinal axis is parallel to the cylinder axis. One edge ofeach strip is in contact with the mandrel, i.e., starts at the insidediameter of the hollow cylinder preform, while the other edge is at theoutside of the preform thereby forming the "rosette pattern" notedabove.

c) The preform is then "debulked" by placing a plastic bag over theoutside of the preform and evacuating the air from the preform materialthereby compacting it.

d) The preform is then removed from the male mandrel and placed inside afemale mandrel in preparation for curing.

e) The preform in the female mandrel is then completely encased in aplastic bag, placed in an autoclave and cured under 21 kgs/cm² at thecuring temperature of the phenolic resin.

f) The cured preform composite is removed from the female mandrel andbaked at 650° C.

g) The baked preform is cut into parallelepiped specimens each about5.08 cm long, 2.54 cm wide and about 0.76 to 1.27 cm thick.

h) the specimens are then rebaked at 950° C. prior to impregnation.

The six preform samples are placed in an evacuatable steel retort andcompletely covered with 250 grams of the poorly graphitizing pitch bysprinkling the pitch in granular or powdered form over and around thesamples such that the height of the granulated pitch is about 1.9 cm,which is about 1 cm above the surface of the parallelepiped compositeblocks. The entire assembly is then placed in a Lindberg box furnace andheated at a heating rate of about 1.3° C./minute to a temperature of305° C. in a vacuum of about 2 cm Hg. After reaching the temperature, 2atmospheres of nitrogen are introduced into the retort and the assemblyis held at that temperature for an additional 2.5 hours.

The furnace is then cooled to room temperature and after scraping offthe excess pitch on the surfaces of the specimens, the impregnatedpreforms are weighed. It is determined that an average of about 24.1% byweight of pitch has been impregnated into each of the specimens.

The preforms are then baked in the Lindberg furnace, by heating at arate of 0.5° C./minute to about 560° C. under 2 atmospheres of nitrogenand held at that temperature for 1 hour. By weighing the baked preforms,it is determined that carbon yield of the retained pitch is 71.7%.Accordingly, the baked preforms had picked up an average of 17.3% byweight of carbon (71.7%×24.1%).

The impregnated preforms are then carbonized in the Lindberg furnace byheating at a rate of 1° C./min to a temperature of 800° C. at whichtemperature it is held for 1 hour. The carbonized preforms have anaverage density of 1.32 g/cc and a flexural strength of 14,250 psi.

EXAMPLE 12 (Comparison Example)

For comparison purposes, six parallelepiped specimens are impregnatedwith petroleum pitch having a softening point of 120° C. 250 grams ofpowdered pitch is sprinkled onto the preform in the same manner asdescribed in Example 11 and heated to 220° C. under a vacuum of 2 cm Hg.at a heating rate of about 1.5° C./minute and held at the temperaturefor 0.5 hour under vacuum as described in Example 11. Two atmospheres ofnitrogen are then introduced and each of the samples heated for anadditional 0.5 hour.

After cooling, it is determined that an average of about 21.3% by weightof pitch has been impregnated. The preforms are then baked in the samemanner as in Example 11 by heating at a rate of 0.5° C./minute under 2atmospheres of nitrogen to temperature of 555° C. and held at thattemperature for 1 hour. The carbon yield of the impregnant pitch isdetermined to be 69.7% by weight. The net weight percent pick-up is14.8% by weight (69.7%×21.3%).

The preforms are then carbonized by heating at a rate of 1° C./minute toa temperature of 800° C. and held at that temperature for 1 hour.

The resulting carbonized preforms had an average density of 1.32 g/ccand a flexural strength of 13,620 psi.

Thus, although the preforms in this comparison example and the preformsof Example 11 have the same density after carbonization, the averageflexural strength of the preforms prepared with a conventional pitch isstill less than the flexural strength of the composites prepared withpoorly graphitizing pitch as an impregnant.

What is claimed is:
 1. A process for the production of a unitarycomposite structure comprising a heterogeneous combination ofcarbonaceous reinforcing material interbonded with a matrix materialcomprising:(a) providing the matrix material by treating a tar or apitch having aromatic components with a cross-linking agent topolymerize and cross-link the aromatic components to the extent that thepitch becomes poorly graphitizing and forms substantially no mesophasepitch or forms mesophase pitch having a domain size of less than about30μ; (b) combining the poorly graphitizing matrix pitch material withreinforcing material; and then (c) subjecting the combination ofreinforcing material and matrix material to carbonization conditions toform the interbonded unitary composite structure, wherein the tar orpitch is polymerized and cross-linked by being contacted with at leastone Lewis acid selected from the group consisting of FeCl₃, SnCl₂,SnCl₄, FeBr₃, I₂, ZnCl₂, SbCl₃, SbCl₅, CoCl₂, BF₃ and combinationsthereof.
 2. The process of claim 1, wherein after polymerization andcross-linking, the Lewis acid components are removed by solventextraction.
 3. A unitary composite structure comprising a heterogeneouscombination of a carbonaceous reinforcing material interbonded with amatrix material, said matrix material being a carbonaceous pitchcross-linked with oxygen containing polymerized and cross-linkedaromatic components thereof which have been cross-linked to the extentthat the pitch is poorly graphitizing and has formed substantially nomesophase pitch or contains mesophase pitch having a domain size of lessthan about 30μ.
 4. The composite structure of claim 3, wherein thecross-linked pitch comprises between about 2% and 12% by weight ofoxygen cross-linked with the pitch.